The following steps solved my problems:
Created a default user and set everything up the way I wanted to. Configured bookmarks in my browser, added the local Jabber account, Shortcuts, Panels etc.
Copied the config folders and files like e.g. ~/.mozilla for firefox or ~/.purple for pidgin and the ~/.config/xfce4 directory for the XFCE setup into /etc/skel/ where all files are located that will be copied to a new user's home directory.
Replaced every time the username of my default user occured in the copied config files with ##USERNAME_REPLACE##
Created /usr/local/sbin/adduser.local what is automatically executed when "adduser" is called.
Wrote some substitution bash scripts that replace ##USERNAME_REPLACE## with the name of the newly created user. (Parameter $1 when adduser.local ist called by adduser.)
Because I use apache2-mpm-itk I need to add a vhost to the webserver configuration and one subdomain (username.localhost) per user to the /etc/hosts. Therefore I wrote a bash script that does the trick.
Finally after I set up the whole automation scripts they had to be called from adduser.local with the username (Parameter $1) of newly created users.
Everytime I call e.g. "adduser test_user" adduser.local is executed and does a wonderful setup so I can relax until everything is in place.
Important for me was, that adduser.local does not have to be a pearl script like the example files found on the system.
Hope it helps some others though.